There are at least two different ways to look at American entry into this war.
First, we can say that it came about for idealistic reasons. President Wilson said that it was a war “to make the world safe for democracy.” A major influence on the US decision to enter the war was the fact that Germany was violating international law through its use of unlimited submarine warfare. The US wanted to make sure that international law and the rights of neutral countries were respected. From this point of view, the US entered the war for good reasons.
Second, we can say that it came about for much less idealistic reasons. We can point out that England was also violating international law by blockading Germany. From this point of view, the real reason that we entered the war was because it was in our economic interests to do so. First of all, we had much more trade with England than with Germany, particularly after the war started and England blockaded Germany. Second, the British owed us a great deal of money by the time 1917 came around and we wanted to make sure they won the war so they could pay us back. This makes our entry into the war seem much less principled.